Effects of electromigration on the growth of intermetallic compounds in Cu/SnBi/Cu solder joints

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In this study, the effects of electromigration (EM) on the growth of Cu–Sn intermetallic compounds (IMCs) in Cu/SnBi/Cu solder joints under 5 × 103 A/cm2 direct current stressing at 308, 328, and 348 K were investigated. For each Cu/SnBi/Cu solder joint under current stressing, the IMCs at the cathode side grew faster than that at the anode side. The growth of these IMCs at the anode side and the cathode side were enhanced by electric current. The growth of these IMCs at the cathode followed a parabolic growth law. The kinetics parameters of the growth of the IMCs were calculated from the thickness data of the IMCs at the cathode side at different ambient temperatures. The calculated intrinsic diffusivity (D0) of the Cu–Sn IMCs was 9.91 × 10−5 m2/s, and the activation energy of the growth of the total Cu–Sn IMC layer was 89.2 kJ/mol (0.92 eV).

I. INTRODUCTION

Lead-free solders have attracted extensive research due to legislative restrictions1 on the use of lead in electronics. Lead-free solders from systems such as SnAg, SnCu, SnAgCu, SnZn, SnBi, and SnIn have been developed as possible candidates to replace the conventional SnPb solder. It is known that SnAgCu ternary alloys possess good solderability and mechanical properties and have been widely used as reliable lead-free solders.2 However, due to the high melting point, around 217 °C, SnAgCu ternary alloys cannot be used for soldering in some cases, such as temperature-sensitive components, optoelectronic modules, in step soldering processes, and thin printed wiring boards (PWBs).3 Eutectic SnBi alloy has a melting point of 138 °C, which is a preferable solder for low-temperature applications.4 With the continuous scaling down of dimensions and increase of current densities in interconnects, electromigration (EM) has become a great reliability concern for microelectronic devices.5 Several types of solder have been studied on several types of under bump metallizations (UBM)6–13 to investigate the failure mechanisms of solder joints under current stressing. However, only a few investigations have been conducted on the interfacial reactions in the solder joints under current stressing.14–17

a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0331 J. Mater. Res., Vol. 23, No. 10, Oct 2008

http://journals.cambridge.org

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Yang and Shang11 have reported EM in eutectic SnBi/Cu solder joints. However, the effect of EM on the interfacial reactions of SnBi/Cu has not been clarified. Thus it is necessary to investigate the effects of EM on the growth of Cu–Sn IMCs in SnBi/Cu solder joints. In this study, the effects of EM on the growth of Cu– Sn intermetallic compounds (IMCs) in Cu/SnBi/Cu solder joints have been investigated. The growth behavior of Cu–Sn IMCs under different current stressing conditions was studied in detail. Also, the kinetic parameters of the growth of Cu–Sn IMCs were determined. II. EXPERIMENTAL

The samples were made of two Cu wires with a diameter of 320 ␮m and an eutectic SnBi